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Two white dwarfs with a total mass $>1.4\:M_{\odot}$ merge. It may trigger a type Ia SN explosion.

How about the three cases below?

  1. Two low mass WDs merge, total mass $<1.4\:M_{\odot}$.

  2. A low mass WD + a low mass neutron star, total mass $<2\:M_{\odot}$ but $>1.4\:M_{\odot}$.

  3. A low mass WD + a neutron star, total mass $>2\:M_{\odot}$.

If any of the above can result in a SN, have we observed this kind of SN?

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    $\begingroup$ Are you talking direct hit collisions or the more gradual, tight orbit, gradual collisions as they spin around each other thousands of times. The 2nd type of collision is far more common. $\endgroup$ – userLTK Nov 6 '16 at 17:35
  • $\begingroup$ @ltk I have revised the title. A 0.5 wd+ 1.4ns should be common. $\endgroup$ – questionhang Nov 7 '16 at 4:26
  • $\begingroup$ @questionhang I guess you're referring to the latter? $\endgroup$ – Sir Cumference Nov 7 '16 at 13:19
  • $\begingroup$ @sir yes, in binaries. $\endgroup$ – questionhang Nov 7 '16 at 14:00
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I don't think any of those would give a supernova, because the first two don't cross an important mass boundary, and the last one might make a black hole from a neutron star, but that's not going to be much of a supernova because it's already very compact and there's not going to be much to have a core bounce off of, nor is the white dwarf massive enough for a type Ia. They should all produce some significant emission, depending on how fast the mass is introduced. Perhaps somewhere between what we would call a supernova, and something more along the lines of a classical nova (for the first one) and quasiperiodic X-ray flaring (for the next two). They could get pretty bright, and I don't know if the nomenclature has individual names for each one of those, but they don't quite sound like supernovae.

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